Light emitting diodes (LEDs) are typically monochromatic semiconductor light sources, and are currently available in various colors from UV-blue to green, yellow and red. Many lighting applications such as backlighting for liquid crystal display (LCD) panels require white light sources. Two common approaches for producing white light using monochromatic LEDs include (1) packaging individual red, green, and blue LEDs together and combining the light that is emitted from these LEDs to produce white light and (2) introducing fluorescent material into a UV, blue, or green LED so that some of the original light emitted by the semiconductor die of the LED is converted into longer wavelength light and combining the longer wavelength light with the original UV, blue, or green light to produce white light. LEDs produced using the second approach often use phosphor-based fluorescent material and are referred to as phosphor-converted white LEDs.
White light produced by a combination of red, green, and blue LEDs has a wide color gamut but generally has a poor Color Rendering Index (CRI). Although the color gamut of this type of white light source is wide, the light source requires more complex driving circuitry than a phosphor-converted white LED because the red, green, and blue LEDs include semiconductor dies that have different operating voltage requirements. In addition to having different operating voltage requirements, the red, green, and blue LEDs degrade differently over their operating lifetime, which can make color control over an extended period difficult.
Phosphor-converted white LEDs require only a single type of monochromatic LED to produce white light and all of the white LEDs of a multi-LED light source can be driven at the same operating voltage. One shortcoming of phosphor-converted white LEDs is that their spectral power distribution (SPD) is not uniform. This shortcoming results in a relatively poor Color Rendering Index (CRI). In addition, the color generated by phosphor-converted white LEDs tends to vary from its original value over the operating life and with changes in conditions.
What is needed is a white light source that has a high CRI and a wide color gamut that can produce consistent white light over time.